Method of producing a fully-killed steel with rimming characteristics



United States Patent 2,885,752 METHOD or PRODUCING); i ijLLYrkrLILEDSTEEL wrrr-r RIMMINGCHARACTERISTICS 8 Claims. 01.22 216 This inventionrelates to the production of fully-killed steels and more particularlyto 'anstenitic-chromium manganese fully-killed steel's-havingirnprovedcharacteristics resulting from'the evolution of gas therefrom afterteeming.

The chemical characteristics of the alloying elements involved andeconomical considerations make it very desirable to make allhigh-alloyand most alloy steels following conventional practice offullykilling them, i.e. fully deoxidizing the metal by adding to itappropriately selected elements, such as aluminum and silicon, having agreater aflinity for oxygen than iron. Such elements combine with oxygendissolved in iron forming compounds largely incapable of takingpartinany subsequent reactions. Thus noga's is evolvedwhen a killedsteelsolidifies after teeming into ingot molds. As a result, duringsolidification the steel shrinks with the--formation of internalshrinkage cavity. Occasionally such pipe may be tolerated, as in makingseamless piping or transformer grades silicon steel sheets, but, as ageneral rule, it must be eliminated somewhere in'theprocessing cyclegenerally by cropping the ingot.

The usual and most widely-used methodlfor accomplishing pipe eliminationin'killedsteels consists inplacing above the ingot molds a hotto'p,namely a container possessing lesser heat abstracting characteristicsthan the metal of the ingot mold and which permits a portion of themetal, usually 10 to 20%, to remain liquid for a longer time than thebody of ingots and to feed molten metal into the pipe cavity producedduring solidification.

With the use of properly designed hottops, all pipe is concentratedtherein and is removed by cropping the hot top after the solidificationis completed. Cropped hot tops are scrapped which, in caseof high-alloysteels, exemplified by stainless, high-speed"and similar alloys, causesheavy financial losses.

-In certain low-carbon grades of steel, it'is common practice to makesteel by rimming practica ie. withat least sufficient evolution ofgas,largely carbon monoxide, during solidification to at least preventshrinkage and pipe formation during freezing. In addition.top'errnitting use of a very high percentage of the total ingot and theelimination of hot topping, this results in a rim of essentially puresteel uncontaminated by the'various inclusions appearing in all steels.Thisrim has very beneficial elfects during rolling,sincetroublesomeseams, etc. are largely eliminated. Inkilledste'els dueto the killing agent combining with the avialabl e oxygen, no rimmingcan take place and such desirable "results thereof have hitherto notbeen obtainable. I

It is accordingly'an object of this invention toprovide a method offorming killed steel ingots having the cha'racteristics of rimmedsteels. s s i s It is another object of this invention to ieliniinatethenecessity of hot-topping fully-deoxidizd' alloysteel ingots.

I achieve the foregoing objects, by introducingsufllcient inert gas intothe metal while molten andprior to teem. ing to promote a rimming actionthereinand ftirrna'tiort 2 of gas-filled cavities in the metal duringits solidification to otfset; the decreased volumeof the metal due tocooling and solidification thereof.

Thepreferred gas for practicing my invention is nitrogen which Iintroduce into the molten bath as a solid compound such asnitrogen-bearing salt, cyanamide, or a metallic nitride. While'adequatenitration has been recorded with the use of all these additions, theemployment of metallic nitrides, particularly of chromiumnitride in caseof stainless steels, introduces additional advantages favoring theiruse. For example, high nitrogen ferro-chromium may be used in chromiumalloys or high nitrogen-manganese or term-manganese in manganese alloys.Nitrogen may also be added in the form of a gaseous compound, such asammonia.

A close control of the gas evolution in the present process is possibleonly when the body of the metal isfree from dissolved oxygen capable ofgenerating carbon monoxide onsolidification. With this object in view, athorough killing of the steel to be treated before addingnitrogen-bearing substances is desirable, and can be accomplished in anumber of conventional ways such as those hereinbefore mentioned. Thiscomplete killing is of course particularly desirable in the highalloysteels such as austenitic-chromium-manganese steels. Neitherthekind of deoxidizers used nor the sequence of their addition appear tobear on the efliciency of the present process, provided the deoxidationreaction is properly effected.

It has been found that mere addition of nitrogen to steel, which is wellknown, cannot accomplish the results contemplated by this invention. Inorder to be effective, the gas has to be introduced in closelycontrolled percentages depending, among others, on the composition ofthe steel treated and teeming method used. It is essential that theamount of nitrogen added should be in excess of the solubility ofnitrogen in the particular steel at its melting point. Thisprovides avolume of evolved gas sufficient to impart to the steel the propertiesranging from those wherein the shrinkage cavity is merely replaced bynumerous voids in a random distribution to those of fully-rimmed steelwherein the evolution of gas from the steel is sufiicient to permit theformation of an external rim of pure metal. 7

Extensive tests have demonstrated that liquid pure iron dissolves amatter of 0.040-0.045% nitrogen at 'its melting point, and can retainbut 0.0l0-0.015% nitrogen on solidification. This can be stated that themaximum percentage of nitrogen which can be incorporatedin pure ironcannot exceed 0.045% nitrogen and that adding more than 0.015% nitrogento it will cause formation of blow holes in castings on solidification.Other tests have shown a very pronounced difference'in solubility of thegas caused by alloying pure iron with other elements, and the digressionof this solubility from that expected from the combined effect ofindividual elements alloyed with the iron.

I have discovered that a proper balancing of gas evolution rate onsolidification and the amount of nitrogen added to the molten steel is afunction of steel compositionand can be expressed by appropriateformulas empirically derived in each individual case.

Taking as an illustration a group of austenitic'alloys containing 16-20%chromium, 4-l4% manganese, 4%, maximum, nickel and not more than 0.20%carbon, one can use for determining the percentage of nitrogen to beadded 'for the desired gas evolution the formula:

Percent Gr+1.75 Xpercent Mn +2Xpercent Ni Percent NThlisifoi-mulaprovides a desired minimum nitrdgen Heat Ferrite at No. MnP S Si Ni Or N: 2,100 F. percent These heats were all cast into 23x35x40inch ingots in bottle-top molds and had suffieient rimming action for avery satisfactory mechanical capping. No particular precautions weretaken in their manufacture and no deviations from the conventional goodsteel-making practice were committed, as it can be seen from thefollowing melting sheet of electric furnace heat 1.

Began charge, 3100 lbs. slab 8:30 Full power on 8:40 100 lbs. lightscrap, liquid pool 10:25 1200 lbs. slabs 11:15 Began feeding 1410 lbs.low-carbon ferrochrome 11:25 200 lbs. ferrochrome pellets 11:55 1000lbs. slabs 12:00 150 lbs. nickel 12:35 150 lbs. nickel 12:45 Finishfeeding low-carbon ferrochrome 12:50 270 lbs. low-carbon ferromanganese12:50 580 ferrochrome pellets, power cut 1:30 Power full 1:32 Power cut1:38 Began to skim slag 1:45 Full power 1:54 380 lbs. electrolyticmanganese, violent reaction 1:55 100 lbs. pig iron 2:03 Began tap 2:17Tapped 2:19

The scrap used consisted of low-metalloid crops, ferrochromium pelletswhich analyzed 62.09% chromium, 5.47% silicon, 2.06% nitrogen, 0.11%carbon, and the electrolytic manganese contained 5.81% nitrogen.

The nitrogen content of these alloys was balanced in accordance with theabove formula so that the desired volume of gas was evolved at a properrate on solidification. The ingots produced were essentially austenitic,both at hot-rolling and room temperatures, and their hot workability wasgood. The physical properties of these ingots were similar, in general,to conventional 17% chromium-7% nickel and 18% chromium-9% nickelalloys, and the elimination of hot tops increased their ingot to slabyield from 70-73% to 80- 90%.

Rimming action in other compositions of steel can be obtained bysimilarly adding sufficient nitrogen to exceed the solubility limitthereof in the particular composition at its melting point. Aconventional 17% chromium stainless steel has been made with goodrimming action as has the so-called Hadfield manganese steel whentreated in accordance with my invention by adding nitrogen above theforegoing solubility limit. By proper experimentation, the above formulacan be modified for each such composition.

It must be emphasized that the formula presented is a ready indicationof the minimum amount of nitrogen for a particular grade of steel andmay require modification for adapting the behavior of the metal tooutside conditions, exemplified by dimensions and nature of in gotmolds, and for controlling the internal structure of ingots, as when theformation of an external rim of pure metal is to be preferred to arandom distribution of voids to offset care of volume reduction. In thislatter event about .03 to .04% more nitrogen is required than isrequired for making capped chromium-manganese-austenitic steel, forwhich the formula will determine the proper amount.

While I have shown and described one specific embodiment of myinvention, it will be understood that this embodiment is merely for thepurpose of illustration and description and that various other forms maybe devised within the scope of my invention, as defined in the appendedclaims.

I claim:

1. A method of making fully deoxidized steel ingots having the dense rimand absence of pipe characteristics of rimmed steel comprising forming asteel melt of the desired composition substantially free from dissolvedoxygen, adding nitrogen to said melt in excess of the solubility ofnitrogen in said melt at its melting point, then teeming said nitrogenenriched steel melt into ingot molds and permitting it to solidifytherein, whereby nitrogen in excess of the amount retainable in thesteel in the solid condition evolves as a gas within said melt duringsolidification thereof, promoting the formation of said rim andpreventing the formation of a pipe in the resulting ingots.

2. A method of making fully deoxidized steel ingots having the dense rimand absence of pipe characteristic of rimmed steel comprising forming asteel melt of the desired composition substantially free from dissolvedoxygen, adding nitrogen to said melt in excess of the solubility ofnitrogen in said melt at its melting point, teeming said nitrogenenriched steel melt into ingot molds, capping said molds and permittingsaid melt to solidify therein, whereby nitrogen in excess of the amountretainable in the steel in the solid condition evolves as a gas withinsaid melt during solidification thereof, promoting the formation of saidrim and preventing the formation of a pipe in the resulting ingots.

3. A method of making fully deoxidized austenitic chromium manganesesteel ingots having the dense rim and absence of pipe characteristic ofrimmed steel comprising forming a melt of steel containing 16 to 20%chromium, 4 to 14% manganese, 4% maximum nickel, up to .20% carbon andfree from dissolved oxygen, adding nitrogen to said melt in excess ofthe solubility of nitrogen in said melt at its melting point, thenteeming said nitrogen enriched steel melt into ingot molds, andpermitting it to solidify therein, whereby nitrogen in excess of theamount retainable in the steel in the solid condition evolves as a gaswithin said melt during solidification thereof, promoting the formationof said rim and preventing the formation of a pipe.

4. A method of making fully deoxidized austenitic chromium manganesesteel ingots having the dense rim and absence of pipe characteristic ofrimmed steel comprising forming a melt of steel containing 16 to 20%chromium, 4 to 14% manganese, 4% maximum nickel, up to .20% carbon andfree from dissolved oxygen, adding nitrogen to said melt in accordancewith the formula Percent Cr 1 .75 Xpercent Mn Percent N Xpercent Ni ofmanganese and from three tenths to one percent of nitrogen and in whichthe austenitizing elements and the ferrite forming elements areproportioned to permit a stably austenitic alloy at the desired nitrogenlevel comprising producing a melt of the desired composition, castingthe melt into an ingot mold, permitting a solid skin to form on theingot with the simultaneous ebullition of nitrogen, capping the ingot toentrap a substantial amount of nitrogen in the gaseous form as bubbles,permitting the ingot to completely solidify and subjecting the ingot tointensive hot working whereby the gaseous nitrogen contained in theingot is redissolved and the sides of the nitrogen cavities welded shutto produce a sound and mechanically strong metal.

6. The process of producing a sound iron base, stably austenitic alloycontaining from three tenths of one percent to one percent nitrogen andin which the ferrite forming alloying elements and the austenite formingalloying elements are balanced to produce an alloy which is stablyaustenitic at room temperature and at the desired nitrogen levelcomprising establishing a melt of the desired composition, casting themelt into an ingot mold, permitting a solid skin to form on the ingotwith the simultaneous ebullition of nitrogen, capping the ingot toentrap a substantial amount of nitrogen in the gaseous form as bubbles,permitting the ingot to completely solidify and subjecting the ingot tointensive hot working whereby the gaseous nitrogen contained in theingot is redissolved and the sides of the nitrogen cavities welded shutto produce a sound and mechanically strong metal.

7. An ingot of fully deoxidized steel having the dense 6 rim and absenceof pipe characteristic of rimmed steel ingots.

8. An ingot of fully deoxidized austenitic stainless steel having thedense rim and absence of pipe characteristic of rimmed steel ingots.

References Cited in the file of this patent UNITED STATES PATENTS1,792,967 Clark Feb. 17, 1931 2,181,693 Epstein Nov. 28, 1939 2,236,504Herty Apr. 1, 1941 OTHER REFERENCES Section VIII, Sixth Report, Iron andSteel Institute, pages 137-149 relied on. 1935.

The Making, Shaping and Treating of Steel, US. Steel (6th ed.) 1951.Pages 571-573 and 443-445 relied on.

Section V, Seventh Report of Iron and Steel (1937) Institute onHeterogeneity of Steel Ingots. Pages 139-142 relied on.

The Ingot Phase of Steel Production, 2nd ed., 1942, pp. 22-24 and 43-47relied on.

Section -IV. Principles Involved in the Making of Rimming Steel. FourthReport on the Heterogeneity of Steel Ingots, 1932, pages 68-95.

1. A METHOD OF MAKING FULLY DEOXIDIZED STEEL INGOTS HAVING THE DENSE RIMAND ABSENCE OF PIPE CHARACTERISTICS OF RIMMED STEEL COMPRISING FORMING ASTEEL MELT OF THE DESILRED COMPOSITION SUBSTANTIALLY FREE FROM DISSOLVEDOXYGEN, ADDING NITROGEN TO SAID MELT IN EXCESS OF THE SOLUBILITY OFNITROGEN IN SAID MELT AT ITS MELTING POINT, THEN TEEMING SAID NITROGENENRICHED STEEL MELT INTO INGOT MOLDS AND PERMITTING IT TO SOLIDIFYTHEREIN, WHEREBY NITROGEN IN EXCESS OF THE AMOUNT RETAINABLE IN THESTEEL IN THE SOLID CONDITION EVOLVES AS A GAS WITHIN SAID MELT DURINGSOLIDIFICATION THEREOF, PROMOTING THE FORMATION OF SAID RIM ANDPREVENTING THE FORMATION OF A PIPE IN THE RESULTING INGOTS.